Newer ES features (ES7–ES14): ● includes(), ** (ES7) ● async/await, Object.entries() (ES8) ● Object rest/spread, Promise.finally (ES9) ● Array.flat(), Object.fromEntries() (ES10) ● BigInt, ?

Short answer: ?, ?. (ES11) replaceAll(), Promise.any() (ES12) at(), top-level await (ES13) Array.findLast(), findLastIndex() (ES14+) 🔹 76. includes() method for arrays (ES7) Checks if an array contains a specific element and returns true or false. const fruits = ["apple", "banana", "mango"]; console.log(fruits.includes("banana")); // true… Explain a bit… more…… console.log(fruits.includes("grapes")); // false 🔹 77. async…

Explain a bit more

function fetchData() { try { const response = await fetch(" const data = await response.json(); console.log(data); } catch (err) { console.error(err); fetchData(); Notes: async marks a function as asynchronous and it returns a Promise.

Example code

fetch('/api/data') .then(res => res.json()) .catch(err => console.error(err)) .finally(() => console.log('Fetch attempt finished')); 🔹 83. Asynchronous iterators (ES9) Allow iterating over asynchronous data streams using for await...of. Example: async function* asyncGenerator() { yield await Promise.resolve(1); yield await Promise.resolve(2); yield await Promise.resolve(3); (async () => { for await (const num of asyncGenerator()) { console.log(num); })(); // Output: 1, 2, 3 🔹 84. Array.flat() and Array.flatMap() (ES10) Array.flat(depth) – Flattens nested arrays up to the specified depth. Example: const arr = [1, [2, [3, 4]]]; console.log(arr.flat(1)); // [1, 2, [3, 4]] console.log(arr.flat(2)); // [1, 2, 3, 4] Array.flatMap() – Maps each element and flattens the result by one level. Example: const arr = [1, 2, 3]; const result = arr.flatMap(x => [x, x*2]); console.log(result); // [1, 2, 2, 4, 3, 6] 🔹 85. Object.fromEntries() (ES10) Converts an array of key-value pairs into an object. Example: const entries = [['name', 'Sandeep'], ['age', 30]]; const obj = Object.fromEntries(entries); console.log(obj); // {name: "Sandeep", age: 30} 🔹 86. Optional catch binding (ES10) Allows omitting the error parameter in catch if it’s not needed. Example: try { throw new Error("Oops"); } catch { console.log("Error handled without using the error object"); 🔹 87. BigInt (ES11 / 2020) Represents integers larger than Number.MAX_SAFE_INTEGER. Example: const bigNumber = 123456789012345678901234567890n; console.log(bigNumber + 1n); // 123456789012345678901234567891n Notes: Use n at the end to denote a BigInt literal. Cannot mix Number and BigInt directly in arithmetic. 🔹 88. Nullish coalescing operator ?? (ES11 / 2020) Returns the right-hand side value only if the left-hand side is null or undefined. Example: const name = null; console.log(name ?? "Default Name"); // "Default Name" const age = 0; console.log(age ?? 18); // 0 (not null or undefined) 🔹 89. Optional chaining ?. (ES11 / 2020) Safely accesses nested object properties without throwing an error if a property is null or undefined. Example: const user = { profile: { name: "Sandeep" } }; console.log(user.profile?.name); // "Sandeep" console.log(user.address?.city); // undefined (no error) 🔹 90. Promise.allSettled() (ES11 / 2020) Waits for all promises to settle (fulfilled or rejected) and returns an array of their results. Example: const promises = [Promise.resolve(1), Promise.reject("Error")]; Promise.allSettled(promises).then(results => console.log(results)); // [ // { status: "fulfilled", value: 1 }, // { status: "rejected", reason: "Error" } // ] 🔹 91. replaceAll() for strings (ES12 / 2021) Replaces all occurrences of a substring in a string. Example: const text = "JavaScript is fun. JavaScript is powerful."; console.log(text.replaceAll("JavaScript", "JS")); // "JS is fun. JS is powerful." Note: Unlike replace(), replaceAll() replaces every match, not just the first. 🔹 92. Promise.any() (ES12 / 2021) Returns the first fulfilled promise. Rejects only if all promises are rejected. Example: const promises = [ Promise.reject("Error1"), Promise.resolve(42), Promise.resolve(100) Promise.any(promises).then(result => console.log(result)); // 42 🔹 93. Logical assignment operators (ES12 / 2021) Combine logical operations with assignment: &&=, ||=, ??=. Examples: let a = true; let b = false; // AND assignment a &&= false; console.log(a); // false // OR assignment b ||= true; console.log(b); // true // Nullish assignment let c = null; c ??= 10; console.log(c); // 10 🔹 94. at() method for arrays and strings (ES13 / 2022) Returns the element at a specific index, supporting negative indexing. Example – Array: const arr = [10, 20, 30, 40]; console.log(arr.at(1)); // 20 console.log(arr.at(-1)); // 40 (last element) Example – String: const str = "JavaScript"; console.log(str.at(0)); // "J" console.log(str.at(-3)); // "i" 🔹 95. Top-level await (ES13 / 2022) Allows using await outside async functions in modules. Example: // In a module (ESM) const data = await fetch(" .then(res => res.json()); console.log(data); Notes: Works only in modules, not in regular scripts. Makes asynchronous initialization easier. 🔹 96. Object.hasOwn() (ES13 / 2022) Checks whether an object has a specific own property (safer than hasOwnProperty). Example: const obj = { name: "Sandeep" }; console.log(Object.hasOwn(obj, "name")); // true console.log(Object.hasOwn(obj, "age")); // false Advantage: Works even if hasOwnProperty is overwritten. 🔹 97. Array.findLast() and Array.findLastIndex() (ES14+ / 2023+) findLast() – Returns the last element in the array that satisfies the provided testing function. findLastIndex() – Returns the index of the last element that satisfies the test. Example – findLast(): const arr = [1, 2, 3, 4, 5, 6]; const lastEven = arr.findLast(x => x % 2 === 0); console.log(lastEven); // 6 Example – findLastIndex(): const arr = [1, 2, 3, 4, 5, 6]; const lastEvenIndex = arr.findLastIndex(x => x % 2 === 0); console.log(lastEvenIndex); // 5 🔹 98. New built-in methods introduced recently (ES14+ / 2023+) Some useful new methods in modern JavaScript: Array.findLast() & Array.findLastIndex() – Already covered. Array.toSorted() – Returns a sorted copy of the array without mutating the original. const arr = [3, 1, 2]; const sortedArr = arr.toSorted(); console.log(sortedArr); // [1, 2, 3] console.log(arr); // [3, 1, 2] Array.toReversed() – Returns a reversed copy without mutating original. Array.toSpliced() – Returns a copy of the array with spliced elements removed or replaced. Array.with() – Returns a new array with an element replaced at a given index. String.toSorted(), String.toReversed() – Similar operations for strings. Note: These methods preserve immutability, unlike their older counterparts like sort() and reverse(). 🔹 99. What is the DOM? The Document Object Model (DOM) is a programmatic representation of a webpage, allowing JavaScript to read, manipulate, and modify HTML and CSS. Example: console.log(document.body); // Accesses the <body> element 🔹 100. Difference between innerHTML, innerText, and textContent innerHTML – Returns/sets HTML content, including tags. innerText – Returns/sets rendered text, respects CSS styles. textContent – Returns/sets all text, ignores CSS, faster than innerText. Example: const div = document.querySelector("#myDiv"); div.innerHTML = "<p>Hello</p>"; // Inserts <p>Hello</p> div.innerText = "<p>Hello</p>"; // Displays "<p>Hello</p>" div.textContent = "<p>Hello</p>"; // Displays "<p>Hello</p>" 🔹 101. How do you select DOM elements? Use selectors like: document.getElementById("id") document.getElementsByClassName("className") document.getElementsByTagName("tagName") document.querySelector(".className") document.querySelectorAll("div p") 🔹 102. Difference between getElementById and querySelector getElementById – Selects an element by its ID, returns one element. querySelector – Selects first matching element by CSS selector (ID, class, tag, etc.). Example: const el1 = document.getElementById("myDiv"); const el2 = document.querySelector("#myDiv"); // Same result 🔹 103. How do you create and append DOM elements? const div = document.createElement("div"); div.textContent = "Hello World"; document.body.appendChild(div); // Adds the div to body 🔹 104. How do you update styles using JavaScript? const div = document.querySelector("#myDiv"); div.style.backgroundColor = "lightblue"; div.style.fontSize = "20px"; 🔹 105. What is addEventListener? Attaches event listeners to DOM elements without overwriting existing ones. Example: const button = document.querySelector("#myButton"); button.addEventListener("click", () => { alert("Button clicked!"); }); 🔹 106. What is localStorage? localStorage allows you to store key-value pairs in the browser that persist even after the browser is closed. Example: localStorage.setItem("username", "Sandeep"); console.log(localStorage.getItem("username")); // "Sandeep" 🔹 107. What is sessionStorage? sessionStorage stores key-value pairs for the duration of a page session. Data is cleared when the tab or browser is closed. Example: sessionStorage.setItem("token", "abc123"); console.log(sessionStorage.getItem("token")); // "abc123" 🔹 108. Difference between localStorage and sessionStorage Feature localStorage sessionStorage Lifetime Persist after browser closes Cleared on tab/browser close Storage limit ~5-10 MB ~5 MB Scope Shared across tabs of the same origin Only available in the current tab 🔹 109. How do cookies work in JavaScript? Cookies are small pieces of data stored in the browser and sent to the server with each request. Example: // Set a cookie document.cookie = "username=Sandeep; expires=Fri, 31 Dec 2025 23:59:59 GMT; path=/"; // Read cookies console.log(document.cookie); 🔹 110. Difference between cookies and localStorage Feature Cookies localStorage Sent to server? Yes, with every HTTP request No, only client-side Storage size ~4 KB ~5-10 MB Expiry Can have expiration date Persistent until explicitly removed Accessibility Both client & server Client only 🔹 111. What is the BOM (Browser Object Model)? The Browser Object Model represents the browser’s window and environment, allowing JavaScript to interact with the browser itself (not the page content). Example: console.log(window.innerWidth); // Width of browser window console.log(window.navigator.userAgent); // Browser info 🔹 112. Difference between window and document window – Represents the browser window, contains the BOM, global functions, timers, and document. document – Represents the HTML page content, part of the DOM. Example: console.log(window.location.href); // Current URL console.log(document.title); // Page title 🔹 113. How do you use navigator and location objects? navigator – Provides browser information. console.log(navigator.userAgent); console.log(navigator.language); location – Provides URL info and allows navigation. console.log(location.href); // Full URL location.href = " // Redirects page 🔹 114. What is setTimeout() and setInterval()? setTimeout() – Executes a function once after a delay. setTimeout(() => console.log("Hello after 2s"), 2000); setInterval() – Executes a function repeatedly at intervals. setInterval(() => console.log("Repeating every 1s"), 1000); 🔹 115. How do you detect browser features? Feature detection ensures code runs only if a feature is supported. if ('geolocation' in navigator) { console.log("Geolocation is supported!"); } else { console.log("Geolocation not supported"); 🔹 116. What is a JavaScript module? A module is a file that encapsulates code (variables, functions, classes) and exports it so it can be imported into other files, promoting modularity and reusability. Example (module.js): export const pi = 3.14; export function area(radius) { return pi * radius * radius; Usage (main.js): import { pi, area } from './module.js'; console.log(area(5)); // 78.5 🔹 117. What are named vs default exports? Named exports – Export multiple things with their names; must import using {}. export const x = 10; export const y = 20; // Import import { x, y } from './module.js'; Default export – Only one export per module; can import with any name. export default function greet() { console.log("Hello!"); } // Import import greetFunc from './module.js'; greetFunc(); // Hello! 🔹 118. How do ES6 modules differ from CommonJS? Feature ES6 Modules CommonJS Syntax import / export require / module.exports Loading Static (compile-time) Dynamic (runtime) Browser support Native in modern browsers Needs bundler (Webpack, Node.js) Tree shaking Supported Not supported 🔹 119. What is tree shaking? Tree shaking is a technique to remove unused code from a module during bundling, reducing file size. Example: // utils.js export function usedFunc() { return 1; } export function unusedFunc() { return 2; } // Only import usedFunc import { usedFunc } from './utils.js'; Bundler removes unusedFunc from the final bundle. 🔹 120. What is a memory leak in JavaScript? A memory leak occurs when memory that is no longer needed is not released, leading to increased memory usage and potential slowdowns. Example: let arr = []; function addToArray() { arr.push(new Array(1000000).fill('*')); // Keeps adding large arrays Memory keeps growing because references are not cleared. 🔹 121. How can you avoid memory leaks? Remove event listeners when not needed. Nullify references to large objects. Avoid global variables. Use WeakMap or WeakSet for temporary storage. Properly manage closures. 🔹 122. What is debouncing and throttling? Debouncing – Ensures a function runs only after a specified delay since the last call. Useful for input events. function debounce(func, delay) { let timer; return function(...args) { clearTimeout(timer); timer = setTimeout(() => func.apply(this, args), delay); Throttling – Ensures a function runs at most once every specified interval, useful for scroll or resize events. function throttle(func, limit) { let lastCall = 0; return function(...args) { const now = Date.now(); if (now - lastCall >= limit) { lastCall = now; func.apply(this, args); 🔹 123. What is garbage collection? Garbage collection is the automatic process of freeing memory that is no longer referenced, handled by the JavaScript engine (e.g., V8). Example: let obj = { name: "Sandeep" }; obj = null; // Previous object is now eligible for garbage collection 🔹 124. What is event loop starvation? Event loop starvation occurs when long-running synchronous code blocks the event loop, preventing other tasks (like UI updates or async callbacks) from executing. Example: while(true) {} // Infinite loop blocks everything 🔹 125. What are best practices for writing efficient JavaScript? Minimize DOM manipulations. Use event delegation. Avoid global variables. Use debounce/throttle for frequent events. Leverage ES6+ features for clean and performant code. Use let/const instead of var. Optimize loops and array methods. 🔹 126. What is functional programming? Functional programming (FP) is a programming paradigm where functions are treated as first-class citizens and focus on pure functions, immutability, and avoiding side effects. Example: const add = (a, b) => a + b; // Pure function 🔹 127. What is immutability? Immutability means data cannot be changed after it is created. Instead, new objects or arrays are returned when updates are needed. Example: const arr = [1, 2, 3]; const newArr = [...arr, 4]; // Original arr remains unchanged 🔹 128. What are map(), filter(), and reduce()? map() – Creates a new array by transforming each element. [1,2,3].map(x => x*2); // [2,4,6] filter() – Creates a new array with elements that satisfy a condition. [1,2,3].filter(x => x>1); // [2,3] reduce() – Reduces array to a single value using a reducer function. [1,2,3].reduce((sum, x) => sum+x, 0); // 6 🔹 129. What is function composition? Function composition combines multiple functions into a single function, passing the output of one as input to the next. Example: const double = x => x*2; const increment = x => x+1; const compose = (f, g) => x => f(g(x)); compose(double, increment)(3); // (3+1)*2 = 8 🔹 130. What is currying? Currying transforms a function that takes multiple arguments into a sequence of functions each taking a single argument. Example: const add = a => b => a + b; add(2)(3); // 5 🔹 131. What is partial application? Partial application pre-fills some arguments of a function to create a new function. Example: const multiply = (a, b) => a * b; const double = multiply.bind(null, 2); double(5); // 10 🔹 132. What is a pure function? A pure function always returns the same output for the same input and has no side effects. Example: const square = x => x*x; // Pure 🔹 133. What is referential transparency? Referential transparency means an expression can be replaced by its value without changing the program behavior. Example: const x = 10; console.log(x + 5); // Can replace x with 10 directly 🔹 134. What is the difference between null and undefined? null – Explicitly assigned to indicate no value. undefined – Default value for uninitialized variables or missing properties. let a; // undefined let b = null; // explicitly no value 🔹 135. What is NaN? NaN stands for Not-a-Number, representing an invalid numeric operation. console.log("abc" * 2); // NaN 🔹 136. What is eval() and why is it dangerous? eval() executes a string as JavaScript code. It is dangerous because it can execute malicious code and cause security vulnerabilities. eval("2 + 3"); // 5 🔹 137. What is strict mode ("use strict")? Strict mode enables stricter parsing and error handling in JavaScript, preventing unsafe actions like creating global variables accidentally. "use strict"; x = 5; // Error: x is not defined 🔹 138. What is a Symbol? Symbols are unique and immutable primitive values, often used as object property keys. const sym = Symbol('id'); const obj = { [sym]: 123 }; 🔹 139. What is a generator function? Generator functions use function* and can yield multiple values over time, allowing lazy evaluation. function* gen() { yield 1; yield 2; const g = gen(); console.log(g.next().value); // 1 🔹 140. What is a WeakMap and WeakSet? WeakMap – Object keys are weakly referenced, helps avoid memory leaks. WeakSet – Set of objects with weak references. let wm = new WeakMap(); wm.set({}, 'value'); // Key can be garbage collected 🔹 141. What is tail call optimization? Tail call optimization allows recursive functions to reuse stack frames if the recursive call is the last operation. function factorial(n, acc = 1) { if (n === 0) return acc; return factorial(n - 1, n * acc); // Tail call 🔹 142. Difference between deep copy and shallow copy? Shallow copy – Copies top-level properties only, nested objects remain referenced. Deep copy – Copies entire object structure, no references. const obj = { a: 1, b: { c: 2 } }; const shallow = { ...obj }; const deep = JSON.parse(JSON.stringify(obj)); 🔹 143. What is JSON and how do you parse/serialize it? JSON – JavaScript Object Notation, a lightweight data interchange format. const obj = { name: "John" }; const str = JSON.stringify(obj); // serialize const parsed = JSON.parse(str); // parse 🔹 144. What are template literals? Template literals allow embedded expressions using backticks `. const name = "John"; console.log(`Hello ${name}`); // Hello John 🔹 145. Difference between synchronous and asynchronous code? Synchronous – Executes line by line, blocking further execution. Asynchronous – Executes non-blocking, allows other code to run while waiting. 🔹 146. What are modules and why are they useful? Modules allow splitting code into reusable files and encapsulation, improving maintainability. // math.js export const add = (a,b) => a+b; // main.js import { add } from './math.js'; 🔹 147. What is transpilation (Babel, TypeScript)? Transpilation converts modern JS/TypeScript code into older JavaScript compatible with older browsers. 🔹 148. What is Type Coercion in JavaScript? Type coercion is the automatic conversion between types. console.log('5' - 2); // 3 (string -> number) 🔹 149. How do you detect if a variable is an array? Array.isArray([1,2,3]); // true 🔹 150. What are polyfills? Polyfills implement features in older browsers that don’t support them. if (!Array.prototype.includes) { Array.prototype.includes = function(el) { return this.indexOf(el) !== -1; 🔹 151. How do you prevent global namespace pollution? Use IIFE or modules. Avoid global variables. (function(){ const x = 10; })(); 🔹 152. What are IIFEs (Immediately Invoked Function Expressions)? IIFE – A function that runs immediately after definition, often used for scoping. (function() { console.log("IIFE executed"); })();

Real-world example (ShopNest)

The checkout button uses async/await to call /api/orders, shows a spinner, and catches network errors with a toast.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.